A standard power chuck, such as described in my patent applications Ser. No. 040,126 (now abandoned), Ser. No. 040,433 (now abandoned) and Ser. No. 040,271 (now U.S. Pat. No. 4,275,892) all filed May 18, 1979, has a chuck body that is centered on and rotatable about an axis and that carries a plurality of radially displaceable jaws. These jaws may be split as described in my patent application Ser. No. 159,654 filed June 16, 1980 (now abandoned). Such chucks can be actuated by a compressed-air or hydraulic cylinder system such as described in my applications Ser. No. 090,300 (now U.S. Pat. No. 4,319,516) and Ser. No. 090,348 (now abandoned) filed Nov. 1, 1979 and Ser. No. 208,772 (now U.S. Pat. No. 4,332,186) filed Nov. 20, 1980.
It is standard practice to monitor and even display, for instance via a hydraulic-pressure gauge, the force exerted by the actuator on the jaws, since in a power chuck force must be applied continuously. This actuator force, however, is only a rough approximation of the force being applied radially by the jaws on the workpiece, mainly due to friction in the force-transmission path between the axially moving actuator and the radially moving jaws. Some 30% of the force exerted by the actuator can actually be lost in transmission, and the amount of such losses varies with lubrication, temperature, cleanness of the chuck, and other factors. In addition when a nonround workpiece is being gripped the readout can be wholly inaccurate as the workpiece can be completely out of engagement with one of the three or four jaws of the chuck while it is pinched between the other two or three jaws.
Similarly systems that measure the actual position of the operating member that moves the jaws give a position readout that can vary from the actual position of the jaws. When the chuck mechanism becomes somewhat worn this arrangement also gives a false reading. In addition in a split-jaw chuck such an arrangement is almost wholly useless.
Devices are known which allow the force the jaws are exerting to be determined during setup, that is when the chuck stationary as it is loaded with a workpiece. Although such an arrangement has some advantages, monitoring during use must still be done by one of the less effective above-described methods.